In manufacturing a semiconductor device, testing equipment is typically used for quality control of a finally produced device. In such testing equipment a test head consists of one or more instruments which perform a variety of tests on the device. The temperature control of these instruments is usually accomplished using either a liquid chiller or an air-cooling system. However such test equipment is typically limited in cooling capacity and unsuitable for cooling high-end, high-performance instruments.
Improved testing equipment would be realized if three main functional requirements of the cooling system could be achieved: high-performance, scalability and compactness. High-performing cooling systems would enable high-end instrument development and could be leveraged to improve reliability of tests conducted. A cooling system that scales according to cooling capacity required by the test head is also desirable. Such a system would allow users of low-power instruments to avoid the cost of excess cooling capacity needed by a test head populated with high-power instruments. Finally, a cooling system that can be integrated into a test head would enable a “tester-in-a-test head” system architecture that is compact and easy to work with.
A test head that uses facility water for cooling purposes presents its own unique challenges. Bio-growth and freezing are typically prevented by the addition of chemicals to the facility water. This can leave traces of glycol or polyglycol behind making the water inappropriate for use with bare aluminum. Facility water is generally cooled to a temperature lower than ambient temperature of a laboratory or manufacturing facility. With relative humidity ranges between 30-60%, the dew point at an extreme temperature-relative humidity combination can be above the entering water temperature. As a consequence, water often condenses on the external plumbing surfaces. Facility water may also be undesirable for direct cooling because of the persistent presence of dissolved salts and particles as well as the threat of corrosion to the plumbing. Thus, facility water has been typically employed as a primary loop fluid while another coolant was employed in a secondary, or process loop. This arrangement is similar to a heat exchanger with a dedicated test head coolant loop. Systems developed by Agilent, Schlumberger, Advantest, LTX and Teradyne are examples of this mode.
Typically, such secondary loops are placed in large and expensive cabinets. These cabinets are bulky, and are usually fixed within a section of the room. As a consequence, such systems are frequently dependent on process loop placement and are expensive, cumbersome arrangements.